Devavratha became Bhishma after he took bhishana pratigya ('terrible oath') — vow of life-long celibacy
https://www.google.com/search?q=bhishma+stops+ganga
https://www.thespiritualscientist.com/?s=Bhishma
https://www.google.com/search?q=Shanti+Parva
https://www.google.com/search?q=Bhishma+Lake